I see. I'll increase the pressure by a small amount and see what happens.
I also found these two articles:
(1):
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4301251/(2):
https://sleeptechnologists.org/article/view/659Basically, facemasks can (1) push the tongue to the back of the throat, causing obstruction, and (2) stop the jaw from being gently push forward by CPAP's pressure gradient. I can confirm that (2) happened to me, as my front teeth on my lower jaw felt a bit sore from being pressed against the back of my upper arch's teeth. The facemask forced my mouth completely shut, and the attempted to force my jaw forward but instead just pressed by bottom teeth against my front teeth.
I can see in either case how FL's can be created: the smaller airway volume requires a greater pressure, or else hypoventilation will occur. I'm going to get an oral appliance to stop mouth breathing and allow my jaw to rest in a comfortable position.